1. Technical Field
The present invention relates generally to electronic circuits, and more specifically, the invention relates to switch mode power supplies.
2. Background Information
A common application for a switch mode power supply is a battery charger. The output power of a battery charger is usually controlled to provide regulated output voltage and regulated output current. The output voltage is regulated between a maximum and a minimum voltage over a range of output current. The output current is regulated between a maximum and a minimum current over a range of output voltage. A feedback signal is used to regulate the output of a switch mode power supply such that the output voltage and output current stay within the specified limits.
The switch mode power supply typically has a fault protection feature that prevents excessive output voltage and/or excessive output current in the absence of a feedback signal. Without this fault protection feature, the loss of the feedback signal could cause the output voltage and/or output current to go high enough to damage the output load (which could be a battery) and/or the switch mode power supply. With this fault protection feature, the absence of a feedback signal typically causes the switch mode power supply to operate in an auto-restart cycle that substantially reduces the average output voltage and/or output current until the feedback signal is restored.
Battery chargers usually exhibit an abrupt transition between the regulated output voltage and the regulated output current. That is, the locus of output voltage and output current plotted in Cartesian coordinates usually has a sharp corner of approximately 90 degrees at the point of transition that corresponds to the point of maximum output power.
The typical practice of designing a battery charger with a sharp transition between the regulated output voltage and the regulated output current can result in a product that costs more than necessary to meet the requirements. A controlled regulated transition from a regulated output voltage to a regulated output current can allow the use of lower cost components.
Low cost circuits that regulate output current typically have loose tolerances. Battery chargers that use such circuits must guarantee a minimum output current at one end of the tolerance range, and they must guarantee a maximum output current at the other end of the tolerance range. The need to consider the addition of tolerances in other parameters can cause the design to be capable of substantially higher power than necessary. Failure to deliver all the power required by the load will cause the power supply to lose regulation and to enter a self-protection mode. Higher power capability typically requires a larger magnetic component or a larger power switch, which raises the cost of the power supply.